Position-responsive servo-control unit



Feb. 24, 1959 H. A. VANDER KAAY 2,874,682

POSITION-RESPONSIVE SERVO-CONTROL UNIT Filed Sept. 18, 1957 4 Sheets-Sheet 1 INVENTOR 5 Henrg Adri anYander Kaay Y BY ATTORNEYS I Feb. 24, 1959 H. A VANDER KAAY 4,

POSITiON-RE'SPONSIVE SERVO-CONTROL UNIT Filed Sept. 18. 1957 4 Sheets-Sheet 2 Fix-3.3

- INVENTOR Henry Adrianvander Kaay ATTORNEYS Feb. 24, 1959 V E KAAY I 2,874,682

Y v POSITION-RESPONSIVE SERVO-CONTROL UNIT Filed Sept. 18, 1957 4 Sheets-Sheet 5 INVENTOR. Henry Adrian Vander Kaay ATTORNEYS Feb. 24, 1959 H. A. VANDER KAAY 2,874,682

POSITION-RESPONSIVE SERVO-CONTROL UNIT Filed Sept. 18, 1957 4 Sheets-Sheet 4 v Tic-3. 8

INVENTOR Henrg Adrian Vander Kaag M31 in ATTORNEYS United States Patent "ice POSITIGNPRESPONSIVE ssavo-comnor. UNIT Henry Adrian Vander Kaay, Kalamazoo, Mich., assignor to The New York Air Brake Company, a corporation of New Jersey Application September. 18, 1957, Serial No. 684,745 8 Claims. 01. 121-41 This inventionrelates to fiuid+pressure motors and par.- ticularly to servo-motors operated by hydraulic liquid; A common use for suchmotors is tooperate maneuvering gear om vehicles. This may take various forms. such as an elevating gear, a steering gear, loading and unloading devicestoname a few. Such motors usually are doubleacting.

Variable displacement pumps being expensive,. heavy and-diificnltto maintain, and sinceaccumulators are objectionable for this class of service, reliance is often placed ona constant delivery pump supplyingranopencenter distributing valve. In such a system itis possible to use; an. additional unloader local to the pump. This minimizes heating at no load, but even so, graduated con trol. depends on throttling of the full output of the pump whichwastes energy and results in heating. Graduated control of the output of a large pump is unsatisfactory because the controlled device tendsto overtravel.

Another scheme, long used extensively in the hydraulic art, operates two pumpsin parallel and includesmeans to unload both or only one, or neither. The pump or pumps, when not unloaded, are operated: at full load. The pumps are commonly of different capacity and usually are run at equal. speeds. Where; wide range progressive. speed graduation of the hydraulic motor. is not vital, such a scheme is-satisfactory and economical, for throttling is not the basisof control.

The. present. invention takes advantage: of the economy permittedby. using two pumps,.and.adds graduation of speed secured by throttling. Thettwo pumpsare of widely ditterent capacity, and flow from the smallcapacity pump is controlled directly by a maneuvering valve, preferably of the. open-center type. This permits accurate graduation. effected by throttling a small flow. The energy losses are small. The larger pumpis initially'completely unloaded Its delivery is controlled by another'valve, preferably of theopen-center type and similar incharacterto the maneuvering valve, and piloted by the;maneuveringvalvein such a way that it parallelsthecflow-com trolling.v functions. of the maneuvering valve. but at a larger flow rate, and in a later phase of control. This later phase starts whenthe maneuvering valve haspassed through the control rangev for. the entire output of the first pump, and moves into an additional range affording control of a piloted motor which actuates the: second valve.

The'maneuveringvalve is positioned bythe action of an operator-set elementand a follow-up. controlres'pom sive to motor movement. Hence as the motor approaches the desired. position, determinedsby the operator-set ele ment, the. piloted valve: is returned. to neutral and the.

final approachiscontrolled by' thezgraduating action of the'maneuvering valvewhich throttles only thefl'ow from preferredembodimentofthe invention will nowbe E f follow-up linkage.

2,874,682 Patented Feb. 24,-- 1959 8d describedxby reference to the accompanying drawings, in which:

Figs; 1 and 2 are respectively an elevation and a plan of the. dual valve unit, these views being used to locate essential external connections and to define planes of section'for otherv views.

Fig; 3 is a section taken on a plane definedby theatres of the maneuvering valve plunger and the piloted-valve plunger. The scale is larger than that of'Figs; 1 and 2. The plane of? section. is indicated by the line 33 of Fig. 2. Both:v valves are shown in their centered or lap position in which positionports to the controlled motor are closed.

Fig, 4 is a section on the axis of the maneuvering valve plunger on? a plane at 90 to the plane of section of Fig. 3? and at the same scale as Fig. 3; The line 4'-4 on. Fig. 1 willassistf in identifying the plane of section.

Fig. 5- is' a fragmentary section on the line 5-5 of Fig. Zand'at the scale of'Fig. 3.

Fig. 6 is a fragmentary section on the line 6-6. of Fig. 2 and at the scale of Fig. 3.

Fig. 7' is a fragmentaryv section on the line 77 of Fig. 3-and at the scale of that figure.

. Fig. 8- is a diagram of the manual controller, the controlvalve-and thecontrolled motor with a simplified To make the figure clearly legible and keep within the limits of a single sheet, departures from true'scale have been: found necessary. The linkage. per se is of' aknown. type and the view is adequate for. purpose of explanation. 1

v Statements of. direction, when made without. qualifica tion,.refer to Fig. 3.

The principal features of novelty reside in a dual valve unit having two reciprocable plungers, one of which, called :the primary'distributing valve, is'subject tomanual control with folloW-up-eflfected by motion response of the controlled. motor,;and the other of which, called the secondary distributingvalve, is shifted by a pressure motor piloted. by-the primary. distributing. valve when the latter is shifted beyond its initial range of motion in eitherdirection. I

Before describing the.dual valve, a system to whichit is suited-wiIlbe-described by reference to Fig. 8. p

The maneuvering motor. is there shown asv a. piston motor. 11, having. working-spaces 12 and 13 separated. by;

v a piston 14 whose rod 15 is connected tov adeviceto be maneuvered, for. example. a vehicle steering gear (not shown).

, 1 Control isiefiected by a. wheel 16 which through a pinion and". sector unit,1 7. swings lever. 18 about afixed. fulcrum 19, between. limits. indicated. on the drawing. A motioncombininglinlt- 21 is hinged to lever 18 and to suspender link- 22 which ,is carried by'the-short end.

of rock; lever 23.. This. lever .23 is fulcrurned at 24.and.-

connected torodl 15bylink 25.

Link.21-. is connected? by, link- 26 to: the end of. plunger tributing valve, above mentioned.

The system has two sourcesw of pressure fiuid.: o r

dinarily;v an zhydraulic oil) a. small pump1'29*andt a-.largerz pump 31 which are shown as driven by a sin'gleicom a similar function for the larger pump-31".

The= connections "to' the working spaces of motor ll from housing zfi -areindicated at3l6an'd 31in Figs"; l -3;

inclusive, and 36 appears also' 'in'Figz 5: The locations of these two connectionsare indicated inbroken lines 'in' Fig: 3E 'llie mairidischargeconnection'from liousing"28 by reference to Figs. 3, 4 and 5.

Though not strictly necessary, it is convenient to use the illustrated arrangement in which the primary 'distributing valve plunger 27 and the larger secondary distributing valve plunger 41 are reciprocable in parallel seat bores. Each valve is of the open center type so that the discharge porting is similar. This porting difiers little except in size and spacing as between the seats for the two valves, and the illustrated scheme of connecting together similar discharge ports of the two bores and then using one sump connection for all, affords economy in the use of external connections. The same scheme is used for the two motor ports, to the extent that they are connected between the two bores. I

The plungers are of the type disclosed in the Stephens Patent 2,362,944, Nov. 14, 1944, that is to say they are bored and counterborcd from each end to afford separated flow passages each with a valve seat and coacting spring seated check valve eifective to prevent undesired back-flow through the motor ports in the plunger in the event of failure of the motive fluid supply.

An open center valve is one in which the main supply port communicates with the main discharge passage when the valve is in its neutral or mid-position (sometimes called lap position or hold position). For a doubleacting motor these ports are commonly at mid-length of the valve seat and plunger, and such an arrangement is shown as to each of plungers 27 and 41.

The main supply port 42 for the primary distributing valve 27 is an annular groove encircling the primary valve seat boreand is fed by connection 34. It unloads the system into the bifurcated discharge port 35 whose bifurcations 35a, 35b straddle it. Conversely the main supply port 43 for the secondary distributing valve (which is fed by connection 35) has bifurcations 43a, 43b which straddle port 35. The bifurcations 35a and 35b communicate directly with manifold 39 (see Fig. 4). As a result the open center porting is simple and compact and can be formed by casting procedures.

There must be two exhaust passages for discharging working spaces of motor 11. Encircling the seat bore for plunger 27 are annular groove ports 45 and 46, and encircling the seat bore for plunger 41 are annular groove. ports 47 and 48. As clearly shown in Fig. 3, 45 and 47 are connected together by passage 49, as are 46 and 48 by passage 51. As shown in Fig. 4, 45 and 46 are extensions from manifold 39.

Overload relief valves are arranged to open into extensions of passage 49. These extensions are designated as 49a and 49b. The relief valves difier only in size and are of known form. The one which relieves supply port 42 into space 4911 comprises an insert 51 clamped in a bored seat by set screw 52. The insert has at its lower end a dashpot 53 andat its upper end a seat 54 for a poppet valve. Between these parts are side ports 55. The valve unit proper comprises a valve head 56 and a piston 57 connected by an axially ported stem 58, ,the piston 57 working in the dashpot 53. A bonnet 59 threaded into housing 28 carries an adjusting screw 61 for the valve-loading spring 62. A check nut and acorn nut clearly shown in section in Fig. 3 complete the assembly.

A relief valve assembly identical as to form but larger relieves port 43 into extension 49b. Its components are identified by primed numerals (51' to 62'). In a prototype control unit each relief valve was set for appoximately 1350 p. s. i.

Leakage collecting annular grooves 60 encircle the plunger 27 near each end. The upper one is drained to space 49a (see Fig. 3) and the lower to port 46 (see Fig. 4).

Between port 35 and port 49 is the motor-cylinder port 63 (connected with working space 12), and between port 35 and port 51 is the motor-cylinder port 64 (connected with working space 13). Each of these ports terminates at each of its ends in a corresponding annular groove port formed in body 28, the annular grooves 63a and 64a encircling the plunger 27 and the annular grooves 63b and 64b encircling the plunger 41. Just as ports 47 and 48 are more widely spaced than are 45 and 46, the annular grooves 63b and 64b are more widely spaced than are the annular grooves63a and 64a. The purpose is to increase the functional ranges of motion of plunger 41 as compared with plunger 27. Plunger 41 is actuated by a double-acting piston motor of rather small displacement. The functional travel of plunger 41 is large as compared to that of plunger 27 as its travel controls the output of the large pump 31.

As has been stated, plunger 27 is bored and counterbored from each end. This affords a partition 65 bev tween the bores and two seats 66 and 67 for spring loaded poppet valves 68, 69. Each poppet valve is crossbored, as shown. The valve loading springs 71 are confined between respective valves 68 or 69 and spring seats 72. The upper seat 72 is supported by a gasketed member 73 which is threaded into the upper end of the plunger 27 and serves both as a sealed plug and as a shackle for the attachment of link 26 (see Fig. 1). The lower seat 72 is supported by a flanged member 74 which is threaded into the lower end of plunger 27 and serves both as a sealed plug and as a stop coacting with the slidable ring 75 to limit upward motion of the plunger. Ring 75 also coacts with shoulder 70 on plunger 27 and cap to limit downward movement of the plunger. Eachpoppet valve 68, 69 controls outflow through side ports 76 which lead to respective shallow grooves 77 surrounding the plunger. There are also side ports 78 throughthe plunger immediately above and below partition 65. There are also shallow grooves 79 encircling plunger 27 in position to coact with port 45 and port 46, respectively.

- In mid-position of plunger 27, as shown, port 42 unloads into port 35 and ports 63 and 64 are lapped. In an initial narrow range of positions above and below mid-position one of ports 64 or 63 is charged and the other vented to admit pressure liquid to one or the other working space 12, 13 from pump 29.

The plunger 41 is also an open center valve coacting with ports 47, 48, 63, 64, 43a, 43b and 35 to parallel on a larger flow-rate basis, the valve functions of plunger 27. The plunger 41 is bored from its ends and counterbored to afford a middle partition 81 and seats for two poppet valves 82 and 83. These have cross ports and seat respectively adjacent side ports 84, 85 in the tubular portions of plunger 41. The poppet valves are seated by springs 86, 87 and are sustained by combined spring seats and sealed plugs 88, 89 as shown. The lower plug 89 sustains and guides a centering spring unit. The plug 89 affords a cylindrical guide 91 with flange 92 at its lower end. The end of plunger 41 provides a shoulder 93 at the upper end of the guide 91. A helical spring.94 encircles guide 91 and reacts oppositely on two flanged, annular spring seats 95 which are slidable on guide 91. This assembly is confined in a cylindrical recess 96 formed in body 28 and closed by gasketed cap 97. Fig. 3 shows the construction and the proportions which assure centering of valve plunger 41. A gasketed cap 98 encloses the upper end of plunger 41.

From the construction just described, the plunger 41 functions as a movable abutment between two working spaces, 99 at the upper end and 101 at the lower end. Space 99 is connected to a groove port 102 encircling plunger 27, and space 101 is connected to groove port 103 also encircling that plunger. In mid-position of plunger 27 these ports 102, 103 are connected respectively to exhaust ports 45 and 46 by the grooves 79, 79 which ate-74,682

encircle plunger 27. Thus when plunger 27 moves to its midposition plunger- 41 is freed tomovetoits centered position under the urge of'spring 94; V

In p1unger27 are 'side'ports 1'04, 105 so that upon motion of that plunger, beyond theinitial narrow range, pressure liquid continueszto 'flowto the selected port 63 or 64 and also willflow from port 42 (the supply port from the smaller pump)-through the cross bores in-ch eck valve 68 to port 102 to spa'ce 99 ifplunger 27 -is.moved' down, or through the crossbores ingcheck valve 69' to port 103'if plunger 27. is moved up. This causes the plunger 41 to be shiftedin the same direction'as plunger 27, to connect the large capacity pump to motor 11.

It should be noted that during movement-of the wheel 16 to-shift plunger 27 ineither direction, the'follow-up mechanism acts: in response'to motor movement to restore the plunger 27 to its mid-position. Therefore, shifting of valve 41 occurs whenthe rate of wheel movement is greater than the rate ofmotor movementLbyan amount such that plunger27', which is controlled by both the wheel and the follow-up linkage, shifts beyond the initial narrow range. Thus, plunger 41remains in-midposition unless a predeterminedturningrate is exceeded.

If plunger 27 be moved quickly beyond the initial range from mid-position, port 63 or port 64 (according to the direction of shift) would still'be opened, so'that port 42 would supply motor ll with motive-liquid. Consequently port 42. might not have suffic'ientflow of pressure liquid also to charge space 99-or 101'quickly if, for example, the motor 11 were lightly loaded. In; such a maneuver, quick shift ofiplunger 41 is important because it supplies liquid from the large pump 31 to motor 11. To cover this contingency, the back pressure valve 166 shown in Fig. 7 is used. There is one suchvalve in each of ports 63 andt64' as indicated in Fig. 3. This valve 106 controls a seat107 in port 63, the motor port leading to space 12- of motor 11. The. valve is seated by a spring 108 andthe space behind thevalve is vented to exhaust port 49 at'109 (see Fig. 3). The. annular shoulder 110' causes the valve 106 to openin response to reverse or exhaust flow through port 63: An identical valve assembly is used in port 64, the vent passage 109a.leading to port 51, all as shown in'Eig; 3.

The springs 108 of the, two valves in; theprototype valve were each adjusted so'that its valve 106: produces a pressure drop of approximatelyilldp. sti. This proved satisfactory. The effect of thevalvesi106: is to ensure a back pressure at all times which: will be suflicient' to shift plunger 41. T

It will be observed that whenever the valve plungers, 27 and 41 are both in mid-positionvthe'piston 1:4 is 'hydraulically locked. To relieveshocks-it is desirable to permit throttled cross flow above chosen pressures (such as are generated only by shock loading)betweenlworking.

spaces 12 and 13 of motor 11. The structure shown in,

Fig. 6 (not the invention of this applicant). is one-way of accomplishing the result. Passagesz63aand 64a are respectively branches of ports 63 andf'64. Twogspring loaded relief valves 111, 112 are reversely set so as to permit throttled flows in reverse directions between passages 63a and 64a. approximately 1600 p. s. i., a value above ordinary. op-= crating pressure of motor 11.

It will be apparent that the described dual valve affords two rates of motor. moveinefit 'i The lower rate of motor movement is controlled byshifting: of: the plunger 27 within initial narrow ranges in either direction from midposition. Within this rangeonly pump 29 is loaded A higher steering rate is afforded by shifting plunger 27 beyond the initial narrow range which causes plunger 41 to be shifted by hydraulic fluid admitted to motor space 99 or 101. Shifting of plunger 41 in either direction from mid-position causes load-ing of pump 31, pump 29 remaining loaded. This causes more rapid movement of the controlled motor. The follow-up mechanism act- In the prototype. they; opened at ing 'on plunger 27 'causes first plunger 27 and then plunger 41' to bereturned to mid position as the motor 11 reaches its desired position.

The inventive concept is not limited to use of thedescribed preferred embodiment and no limitation to this structure should. be implied except as is expressed in the appended claims.

What is claimed is:

1. The combinationof a main double-acting fluid pressure motor whose action is to be controlled; two sources of pressure fluid, namely, a first source and a second source; a shiftable control member, a primary distributing valve'having a mid-position and ranges of actuating positions which are reached by shifts of initial extent in opposite directions from mid-position and'in which ranges the valve establishes admission-and-exhaust flows from said first source which respectively cause the motor to move inrelatively opposite directions; an actuating and follow-up mechanism interconnecting said primary distributing valve, said control member and said main motor, whereby the position of the main motor is governed by the position of the control member; a secondary distributing valve having a mid-position and ranges of actuating positions respectively on opposite sides thereof, in which it establishes admission-and-exhaust flow paths from said second source, which respectively cause the motor to move in relatively opposite directions; means serving to bias the secondary, distributing valve to its mid position; a double-acting piloted pressure motor for overpowering said biasingmeans and shifting said secondary distributingvalve into one or the other of its ranges, and meansfor. controlling said: piloted pressure motor and comprising exhaust anddistributing ports controlled by the primary distributing valve, the distributing ports be ingrendered effective by relatively opposite shifts greater thanrthe' initial relatively. opposite shifts of *the primary distributing valve to energize the piloted pressure motor in, opposite directions, wherebyv the admission path established by. the. second-valve communicates with the working space of the main motor then receiving fluid through the primary distributing valve, andthe exhaust port serving upon return of said primary distributing valve to a position within said range of? actuating positions to .deenergize the'piloted motor.

2. The combination. of. a maneuvering motor of the double-acting expansible chamber. type; means for supplying pressure fluid at two different flow rates; a maneu vering valve of the admission-and-exhaust type having a mid-position, said valve serving to control said motor by admitting and exhausting fluid from the low rate supply means to and. from selected opposite working spaces of said motor inlresponse to opposite initial range shifts from mid-position; valve controlling means having an operator-set component, a motor-moved component, and a follow-up connection therebetween, said connection serving to move saidvalve to said mid-position by response-of the motor. to change of set of the operatorset component; av second valve of the admission-andexhausttype supplied. with motive fluid by the high-rate pressure-supply means and shiftable in opposite directions from a mid-position in which it closes communication with said'motor to positions in which it admits-pressure fluid to one or the other working space of the motor, while exhausting the working space not so selected; a double-actingmotor for shifting said second valve; and means for controlling the last-named motor, comprising ports in said maneuvering. valve rendered effective only by range shifts greater thanthe initial range shifts from mid-position of the; maneuvering: valve, whereby the admission path established by shifting of the second valve communicates with the maneuvering motor working space receiving pressure fluid from said low-rate supply means.

3. The combination of a main double-acting fluid pressure motor whose action is to be controlled; two sources of pressure fluid, namely, a first source and a second source; a shiftable control member; a primary diStIlbUt? ing valve of the open center type having a mid-position in which it unloads the first source, and ranges of actuating positions which are reached by shifts of initial extent in opposite directions from mid-position, and in which ranges the valve loads the first source and establishes admission-and-exhaust flows which respectively cause the motor to move in relatively opposite directions; an actuating and follow-up mechanism interconnecting said primary distributing valve, said control member and said main motor, whereby the position of the main motor is governed by the position of the control member; a secondary distributing valve of the open center type, having a mid-position in which it unloads the second source, and ranges of actuating positions, respectively on opposite sides thereof, in which it loads the second source and establishes admission-and-exhaust flow paths which respectively cause the main motor to move in relatively opposite directions; means serving to bias the secondary distributing valve to its midposition; a double-acting piloted pressure motor for overpowering said biasing means and shifting said secondary distributing valve into one or the other of its ranges aforesaid; and means for controlling said piloted pressure motor comprising exhaust and distributing ports controlled by the primary distributing valve, the distributing ports being rendered effective by relatively opposite shifts greater than the initial relatively opposite shifts of the primary distributing valve to energize the piloted pressure motor in opposite directions, whereby the admission path established by the second valve communicates with the working space of the main motor then receiving fiuid through the primary distributing valve, and the exhaust portserving upon return of said primary distributing valve'to a position within its range of actuating positions to deenergize the piloted motor.

4. The combination defined in claim 1 in which the first source is characterized by a lower flow rate than the second'source.

- 5. The combination defined in claim 1 in which the two sources have different flow rates, and in which the motive pressure fluid for the piloted pressure motor is derived from the source having the lower rate.

6. The combination of a maneuvering motor of the double-acting expansible chamber type; means for supplying pressure fluid at two different flow rates; a maneuvering valve of the admission-and-exhaust type, having a mid-position in which it closes communication with said motor and unloads the low-rate supply means, said valve serving to control said motor by admitting and exhausting fluid from the low-rate supply means to and from selected opposite working spaces of said motor in response to opposite initial range shifts from mid-position; valve-controlling means having an operator-set component, a motor-moved component, and a follow-up connection therebetween, said connection serving to move said valve to said mid-position by response-of the motor to a change of set of the operator-set component; a second valve of the admission-and-exhaust type, supplied with motive fluid by the high rate pressure-supply means, and shiftable in opposite directions from a midposition in which it closes communication with said motor and unloads the high-rate pressure supply means, to positions in which it admits pressure fluid to one or the other working space of the motor, while exhausting the working space not so selected; a double-acting motor for shifting said second valve; and controlling means for the last-named motor, comprising ports in said maneuvering valve rendered efiective only by range shifts greater than the initial range shifts from mid-position of the maneuveringvalve', whereby the admission path established bylshifting 'ofjthe second valve communicates with the maneuvering; motorworking space receiving pressure fiuidvfrom saidlow rate supply means.

7: The combination of a maneuvering motor of the doubleeacting ,expansible chamber type; means for supplyingpressure fluid at two different flow rates; a maneuvering valve=of-the admission-and-exhaust type having a mid-position, said valve serving to control said motor by admitting and exhausting fluid from the low rate supply means to and from selected opposite working spaces of said motor in response to opposite initial range shifts from mid-position; valve controlling means having an operator-set component, a motor-moved component, and a follow-up connection therebetween, said connection serving to move said valve to said mid-position by response of the motor to change of set of the operatorset component; a second valve supplied with motive fluid by the high rate pressure supply means and shiftable in opposite directions from a mid-position in which it closes communication with said motor to positions in which it admits pressure fluid to one or the other working space of the motor; a double-acting motor for shifting said second valve; and means for controlling the last-named motor, comprising ports in said maneuvering valve rendered effective only by range shifts greater than the initial range shifts from mid-position of the maneuvering valve, whereby the admission path established by shifting of the second valve communicates with the maneuvering motor working space receiving pressure fluid from said low rate supply means.

8. The combination of a maneuvering motor of the double-acting expansible chamber type; means for supplying pressure fluid at two different flow rates; a maneuvering valve of the admission-and-exhaust type, having a mid-position in which it closes communication with said motor and unloads the low-rate supply means, said valve serving to control said motor by admitting and exhausting fluid from the low-rate supply means to and from selected opposite working spaces of said motor in response to opposite initial range shifts from mid-position; valve-controlling means having an operator-set component, a motor-moved component, and a follow-up connection therebetween, said connection serving to move said valve to said mid-position by response of the motor to a change of set of the operator-set component; a second valve supplied with motive fluid by the high rate pressure-supply means, and shiftable in opposite directions from a mid-position in which it closes communication with said motor and unloads the high-rate pressure supply means, to positions in which it admits pressure fluid to one or the other working space of the motor; a double-acting motor for shifting said second valve; and controlling means for the last-named motor, comprising ports in said maneuvering valve rendered effective only by range shifts greater than the initial range shifts from mid-position of the maneuvering valve, whereby the admission path established by shifting of the second valve communicates with the maneuvering motor working space receiving pressure fluid from said low rate supply means.

References Cited in the file of this patent UNITED STATES PATENTS A mn mi nmn .xs, s 

